1. Field of Invention
The present invention relates generally to methods and apparatus for transmitting digital data in cable television network systems. More specifically, the present invention relates to methods and apparatus for locating upstream data channels with the least amount of ingress noise or other interference.
2. Discussion of Related Art
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable TV systems now include fiber optics as part of its signal transmission component. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cablecommuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable headend, i.e. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the headend was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers long. Branching off of these trunk lines were distribution or feeder cables (40% of the system""s cable footage) to specific neighborhoods, and drop cables (45% of the system""s cable footage) to homes receiving cable television. Amplifiers are provided to maintain signal strength at various locations along the trunk line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the headend of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 50 to 800 MHz. Broadcast signals were sent downstream; that is, from the headend of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscribers. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the headend, known as return or upstream signal transmission. Not surprisingly, nor were there provisions for digital signal transmission either downstream or upstream.
In the 1980s, cable companies began installing optical fibers between the headend of the cable system and distribution nodes (discussed in greater detail with respect to FIG. 1 below). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. In many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting systems are known as hybrid fiber-coaxial (HFC) systems. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and of other frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, i.e. a service allowing subscribers to send a signal to the cable system indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be up to 80 kilometers long, whereas a typical coaxial trunk line is about 10 kilometers long, as mentioned above. Prior to the 1990s, cable television systems were not intended to be general-purpose communications mechanisms. Their primary purpose was transmitting a variety of television signals to subscribers. Thus, they needed to be one-way transmission paths from a central location, known as the headend, to each subscriber""s home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep into the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas between 100 to 2,000 subscribers. Although networks exclusively using fiber optics would be optimal, presently cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using only fiber optic cables.
FIG. 1 is a block diagram of a two-way HFC cable system utilizing a cable modem for data transmission. It shows a headend 102 (essentially a distribution hub) which can typically service about 40,000 subscribers. Headend 102 contains a cable modem termination system (CMTS) 104 that is needed when transmitting and receiving data using cable modems. CMTS 104 is discussed in greater detail with respect to FIG. 2. Headend 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108.
Each headend can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the headend and each distribution node. In addition, because cable modems were not used, the headend of pre-HFC cable systems did not contain a CMTS. Each of the fiber nodes 108 is connected by a coaxial cable 110 to duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service up to 500 subscribers, depending on the bandwidth. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
Recently, it has been contemplated that HFC cable systems could be used for two-way transmission of digital data. The data may be Internet data, digital audio data, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide up to a thousand times faster transmission of digital data than is presently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 56,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data may be transferred at speeds of 10 million bps, or more. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
Furthermore, subscribers can be fully connected twenty-four hours a day to services without interfering with cable television service or phone service. The cable modem, an improvement of a conventional PC data modem, provides this high speed connectivity and is, therefore, instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services.
As mentioned above, the cable industry has been upgrading its coaxial cable systems to HFC systems that utilize fiber optics to connect headends to fiber nodes and, in some instances, to also use them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and have a wider bandwidth than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers degrade the signal quality and are susceptible to high maintenance costs. Thus, coaxial distribution systems that use fiber optics have much less need for amplifiers. In addition, amplifiers are typically not needed for fiber optic lines (item 106 of FIG. 1) connecting the headend to the fiber nodes.
In cable systems, digital data is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert digital data to a modulated RF signal and converts the RF signal back to digital form. The conversion is done at two points: at the subscriber""s home by a cable modem and at a CMTS located at the headend. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is fed to the cable modem which converts it to a modulated RF signal (it is helpful to keep in mind that the word xe2x80x9cmodemxe2x80x9d is derived from modulator/demodulator). Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
As mentioned above, cable modem technology is in a unique position to meet the demands of users seeking fast access to information services, the Internet and business applications, and can be used by those interested in cablecommuting (a group of workers working from home or remote sites whose numbers will grow as the cable modem infrastructure becomes increasingly prevalent). Not surprisingly, with the growing interest in receiving data over cable network systems, there has been increased focus on performance, reliability, and improved maintenance of such systems. In sum, cable companies are in the midst of a transition from their traditional core business of entertainment video programming to a position as full service providers of video, voice and data telecommunication services. Among the elements that have made this transition possible are technologies such as the cable modem.
A problem common to all upstream data transmission on cable systems, i.e. transmissions from the cable modem in the home back to the headend, is ingress noise which lowers the signal-to-noise ratio, also referred to as carrier-to-noise ratio of an upstream channel. Ingress noise can result from numerous internal and external sources. Sources of noise internal to the cable system may include cable television network equipment, subscriber terminals (televisions, VCRs, cable modems, etc.), intermodular signals resulting from corroded cable termini, and core connections. Significant sources of noise external to the cable system include home appliances, welding machines, automobile ignition systems, and radio broadcasts, e.g. citizen band and ham radio transmissions. All of these ingress noise sources enter the cable system over the coaxial cable line, which acts essentially as a long antenna. Ultimately, when cable systems are entirely optical fiber, ingress noise will be a far less significant problem. However, until that time, ingress noise is and will continue to be a problem with upstream transmissions.
The portion of bandwidth reserved for upstream signals is normally in the 5 to 42 MHz range. Some of this frequency band may be allocated for set-top boxes, pay-per-view, and other services provided over the cable system. Thus, a cable modem may only be entitled to some fraction or xe2x80x9csub-bandxe2x80x9d such as between 200 Khz to 3.2 MHz. This sub-band is referred to as its xe2x80x9calloted band slicexe2x80x9d of the entire upstream frequency range (5 to 42 MHz). This portion of the spectrumxe2x80x94from 5 to 42 MHzxe2x80x94is particularly subject to ingress noise and other types of interference. Thus, cable systems offering two-way data services must be designed to operate given these conditions.
As noted above, ingress noise, typically narrow band, e.g., below 100 KHz, is a general noise pattern found in cable systems. Upstream channel noise resulting from ingress noise adversely impacts upstream data transmission by reducing data throughput and interrupting service, thereby adversely affecting performance and efficient maintenance. One strategy to deal with cable modem ingress noise is to position the modem""s upstream data carrier in an ingress noise gap where ingress noise is determined to be low, such as between radio transmission bands. The goal is to position data carriers to avoid already allocated areas.
Ingress noise varies with time, but tends to accumulate over the system and is measured at the headend (ingress noise is uniform over the entire wire). In addition, while a particular frequency band may have been appropriate for upstream transmissions at the beginning of a transmission, it may later be unacceptably noisy for carrying a signal. Therefore, a cable system must attempt to identify noisy frequency bands and locate optimal or better bands for upstream transmission of data at a given time.
Block 104 of FIG. 1 represents a cable modem termination system connected to a fiber node 108 by pairs of optical fibers 106. The primary functions of the CMTS are (1) receiving signals from external sources 100 and converting the format of those signals, e.g., microwave signals to electrical signals suitable for transmission over the cable system; (2) providing appropriate media access control level packet headers for data received by the cable system, (3) modulating and demodulating the data to and from the cable system, and (4) converting the electrical signal in the CMTS to an optical signal for transmission over the optical lines to the fiber nodes.
FIG. 2 is a block diagram showing the basic components of a cable modem termination system (item 104 of FIG. 1). Data Network Interface 202 is an interface component between external data sources and the cable system. External data sources (item 100 of FIG. 1) transmit data to data network interface 202 via optical fiber, microwave link, satellite link, or through various other media. A Media Access Control Block (MAC Block) 204 receives data packets from Data Network Interface 202. Its primary purpose is to encapsulate the data packets with a MAC headers containing cable modem addresses according to the MCNS standard. A MAC address is necessary to distinguish data from the cable modems since all the modems share a common upstream path, and so that the system knows where to send data. Thus, data packets, regardless of format, must be mapped to a particular MAC address.
MAC block 204 transmits data via a one-way communication medium to a downstream modulator and transmitter 206. Downstream modulator and transmitter 206 takes the data in a packet structure and modulates it on the down stream carrier using, for example, QAM 64 modulation (other methods of modulation can be used such as CDMA {Code Division Multiple Access} OFDM {Orthognal Frequency Divison Multiplexing}, FSK {FREQ Shift Keying}). The return data is likewise modulated using, for example, QAM 16 or QSPK. These modulations methods are well-known in the art.
QAM requires a relatively high signal-to-noise ratio to work such as 20 db. QPSK, on the other hand, used for demodulating the upstream data, does not require as high a value, and can work at around 15 db. QPSK is also less expensive and requires less processing than QAM. However, the downstream data path is considered less hostile to the signal and sends data at a very high rate thus requiring QAM. The data carried upstream is assumed to require a more rubust format such as QPSK.
It should be noted that optical fibers transmit data in one direction per wavelength and coaxial cables can transmit data in two directions. Thus, there generally is only one coaxial cable leaving the fiber node which is used to send and receive data, whereas there are two optical fiber lines or wave guide from the fiber node to the downstream and upstream modulators.
Downstream Modulator and Transmitter 206 converts the digital data packets to modulated downstream RF frames, such as MPEG or ATM frames, using quadrature amplitude modulation, e.g. 64 QAM, forward error correcting (FEC) code, and packet interleaving. Data from other services, such as television, is added at a combiner 207. Converter 208 converts the modulated RF electrical signals to optical signals that can be received and transmitted by a Fiber Node 210. Each Fiber Node 210 can generally service about 500 subscribers depending on bandwidth. Converter 212 converts optical signals transmitted by Fiber Node 210 to electrical signals that can be processed by an Upstream Demodulator and Receiver 214. This component demodulates the upstream RF signal (in the 5-42 Mhz range) using, for example, 16 QAM or QPSK. It then sends the digital data to MAC 204.
One method of locating better channels for the upstream data carrier is manually monitoring a particular channel and gathering historical data with respect to the noise level on that channel over a certain period of time. The historical data is then used to compile statistics regarding noise level change on that channel. This requires that an engineer or other human operator monitor and gather the data manually to compile the statistics regarding noise on the channel. This method presumes that the noise level on a channel is xe2x80x9cregularxe2x80x9d and can be predicted if sufficient statistics are compiled. Even after this process is automated to some degree in terms of monitoring and data gathering, it assumes that the noise on a channel has a pattern that can be detected. This a faulty and unjustifiable assumption. In reality, the noise level on all channels are random and chaotic. Specifically, if multiple periodic, yet independent, functions drive a single system, its measured output has a chaotic output function. Furthermore, even if it were possible to extract some degree of regularity in noise patterns, this process is cumbersome, expensive, and requires a high degree of human intervention.
Another method, closely related, involves using a device for gathering data and processing statistics one frequency at a time. The device essentially accumulates data, hashes it, and derives statistics using probability functions. Once it finds a low-noise channel, the system changes its upstream carrier to that channel. However, the time between determining a potentially lower noise channel and using that channel is in the range of minutes to hours. Consequently, the data is likely outdated by the time the system uses it for locating an upstream data carrier.
Both of these methods assume that the cable system is relatively static. However, it is evident that noise in the cable system is far more chaotic in nature than static. There are hundreds of sources of noise that can interfere with the carrier-to-noise ratio of the upstream data. For example, time, weather, temperature, electrical conductivity in the atmosphere, people""s habits, and other disparate factors can contribute to ingress noise on an upstream data carrier.
Therefore, what is needed is a reliable and efficient method of locating channels for upstream data carriers that are certain to have lower noise levels than the current channel for upstream data, thereby enabling deliberate and intelligent selection of an upstream data carrier in an inherently chaotic network system utilizing cable modems.
To achieve the foregoing and in accordance with the purpose of the present invention, methods of and apparatus for locating clean channels for upstream data carriers in a cable television system utilizing cable modems are described. The present invention addresses data integrity by monitoring a channel or a group of channels for selecting a clean upstream data carrier. This is done by placing a digital power receiver on a channel to determine whether that channel has less ingress noise than the channel currently being used. If it is determined that there is less ingress noise, the system can switch to the cleaner channel with minimal latency. The power receiver contains a field programmable gate array (FPGA) that can be configured to perform a fast Fourier transformation (FFT) or a finite response filter (FIR). When performing in FFT mode, the FPGA provides a selection of N number of channels that are possible candidates for the upstream data carrier. When in FIR mode, the FPGA acts as an N-channel receiver to monitor N number of channels and to select one channel from the N channels as the upstream data carrier.
In a preferred embodiment, a spectrum analyzer is placed in a cable modem termination system located in a headend of a cable television system. The spectrum analyzer contains a programmable device which is operable in a first mode to receive a signal having an associated frequency spectrum made up of several channels, and determines a noise level for each of the channels. The circuitry is operable in a second mode to receive the signal described above and filter the signal to generate several filtered signals where each filtered signal corresponds to one of the channels. The circuitry is also operable in the second mode to monitor another noise level associated with each of the filtered signals. The channels associated with the filtered signals are selected based on the noise levels determined in the first mode.
In another preferred embodiment, the programmable device contains control circuitry for instructing programmable circuitry to operate either in a first mode in which the device performs a fast Fourier transformation or in a second mode in which the device performs as a finite impulse response filter. In yet another preferred embodiment, the channels selected correspond to filtered signals with noise levels that fall below a pre-established threshold noise level. In yet another preferred embodiment, the channels selected correspond to filtered signals with the lowest noise levels. According to another embodiment, a method of locating a low-noise channel for data transmission from a subscriber to a headend in a cable television system is described. In this method, a spectrum analyzer receives a signal having an associated frequency spectrum comprising several channels. A different signal is generated representative of a noise spectrum associated with the first signal where the noise spectrum is made up of several noise levels such that a noise level corresponds to each of the several channels. A subset of the channels is selected according to the noise levels of the channels. The spectrum analyzer then band pass filters the first signal in the subset of selected channels thereby generating several filtered signals. A second set of noise levels, where each noise level corresponds to one of the filtered signals, is monitored and a channel from the subset of channels is selected according to a second noise level. This selected channel is used for data transmission.
In yet another aspect of the invention, a computer readable medium storing programmed instructions arranged to locate a low-noise channel for transmitting data from a subscriber to a headend is described.
In yet another aspect of the invention, a cable television system capable of two-way transmission of data is described. The cable television system includes a downstream data path originating from a headend to subscribers and an upstream data path from the subscribers to the headend. Also described are a downstream modulator and transmitter for transmitting a modulated downstream signal, and an upstream demodulator and receiver for receiving and demodulating the upstream signal. Both are located in a cable modem termination system in the headend. Also included in the cable television system is a spectrum analyzer for receiving and monitoring the upstream signal carried on the upstream data path. The spectrum analyzer comprises signal processing circuitry for generating another upstream signal representative of a noise spectrum associated with the first upstream signal. The noise spectrum is made up of several upstream noise levels, such that a first upstream noise level is associated with each of the several channels. The signal processing circuitry also determines a second upstream noise level for each selected channel from the group of channels. The system selects a channel based on the upstream noise levels. The spectrum analyzer also contains circuitry for controlling operation of the signal processing circuitry.
A further understanding of the nature and advantage of the present invention may be realized by reference to the remaining portions of the specification and drawings.